Hitherto, among conventional light-scattering-type smoke detectors, there are those which in order to keep an electrical circuit portion hermetically sealed, the detecting portion is constituted so that a printed circuit board is introduced into the head part of the cylindrical detector having a cover, a dark box provided with light projecting and light receiving elements as well as a labyrinth is put over the head part to be screwed thereto and fixed by adhesives, and after a dark box cover is mounted on the dark box the dark box is surrounded by a detector cover.
The conventional smoke detector having such a constitution is advantageous in that its overall construction can be made compact, the constitution is simple and it can be hermetically sealed from the outside. However, since the dark box is secured to the head part by adhesives it is not possible to easily inspect or repair the detector by disassembling it once it has been assembled. There are other problems in that inspection of the electrical circuit can not be easily carried out.